Old fashioned Cold War weapon aimed at modern problem

The expulsion of diplomats was a weapon used regularly in the cold war when tit-for-tat exchanges were as common as genuine spy swaps. It was all part of the game - Russia and Britain were spying on each other and both sides knew it. They still do.

While expulsions have been widely applauded at home, they have been largely symbolic. With the exception of the expulsion of more than 100 Soviet officials by the Heath government in the 1970s, they have had no lasting impact and little concrete effect. The security services estimate that there are about 30 Russian intelligence officers based in London, the same number as at the end of the cold war. Only a few are officially declared. The four embassy staff expelled yesterday are said to be among those believed to be spies.

A former USSR ambassador to the UK, Viktor Popov, once said that during the 1980s only 15 "genuine" diplomats were part of the 70 diplomatic staff. Russia had built up a network of so-called moles in Britain during the 1930s-1950s, epitomised by the likes of Philby, Burgess, Maclean and Blunt.

The last time Britain and Russia were embroiled in an espionage row was in January last year. At a time when President Putin was trying to justify restrictions on the activities of western NGOs in Russia, Moscow television showed film purporting to be of British agents hiding communications equipment in a fake rock. The Kremlin threatened to expel four British officials they named as spies. Britain said it would "reciprocate". Nothing happened.

Ten years earlier, in May 1996, Russia expelled four British diplomats it claimed were running a spy ring. Britain responded by throwing out four Russians. Whitehall called it a "four-all draw". Norman MacSween was named as an MI6 officer but stayed in Moscow. Nigel Shakespear, a British businessman who had been expelled before while serving as a military attache in Moscow, was deported in 1996.

In 1995 Aleksandr Malikov, a Russian television journalist, was asked to leave Britain after four years in London. In April 1994, Britain expelled a Russian diplomat after Moscow expelled John Scarlett, the current head of MI6, then the agency's station chief in the Russian capital.

The expulsion of 105 Soviet diplomats, journalists and trade representatives by the Heath government in 1971 is the single biggest action taken against Moscow by any western government. Sir Alec Douglas-Home, then foreign secretary, was accused by Labour of over-reaction. Many were exposed by Oleg Lyalin, the first Soviet intelligence agent to defect since the second world war. Oleg Gordievsky, who later defected from the KGB, said the explusions dealt a heavy blow to the Soviet Union's intelligence effort here. Moscow responded by expelling 18 British embassy staff from Moscow.

From time to time, Russian courts have convicted Russians for spying for Britain, as well as for other western countries. This time is different; Andrei Lugovoi may be a spy, but he is wanted for murder.